The strain effects of ZnSe epilayer on ZnSe/GaAs heterojunction structure grown by molecular beam epitaxy have been investigated by using electroreflectance (ER) spectroscopy. The ER measurements were performed as a function of modulation voltage, dc bias voltage, and temperature. From the room temperature ER spectrum, we observed a heavy-hole (HH: 2.609 eV) and light-hole (LH: 2.628 eV) transitions due to a compressive strain. With increasing the bias voltage, the amplitude of HH transition signal decreased and the amplitude of LH transition signal was almost the same. From the temperature dependence of ER spectra, we have studied the interaction between the strain and the thermal expansion coefficient.